Two-Way Passive Beamforming Design for RIS-Aided FDD Communication Systems

TL;DR

This paper proposes a joint optimization method for RIS-aided FDD systems that enhances both uplink and downlink rates simultaneously, outperforming heuristic approaches through a manifold optimization algorithm.

Contribution

It introduces a novel two-way passive beamforming design for RIS in FDD systems, optimizing for both directions simultaneously with an efficient manifold optimization approach.

Findings

The proposed method significantly enlarges the achievable rate region.

Manifold optimization outperforms heuristic designs.

Phase-averaging is better than time-sharing with many RIS elements.

Abstract

Reconfigurable intelligent surfaces (RISs) are able to provide passive beamforming gain via low-cost reflecting elements and hence improve wireless link quality. This work considers two-way passive beamforming design in RIS-aided frequency division duplexing (FDD) systems where the RIS reflection coefficients are the same for downlink and uplink and should be optimized for both directions simultaneously. We formulate a joint optimization of the transmit/receive beamformers at the base station (BS) and the RIS reflection coefficients. The objective is to maximize the weighted sum of the downlink and uplink rates, where the weighting parameter is adjustable to obtain different achievable downlink-uplink rate pairs. We develop an efficient manifold optimization algorithm to obtain a stationary solution. For comparison, we also introduce two heuristic designs based on one-way optimization, namely, time-sharing and phase-averaging. Simulation results show that the proposed manifold-based two-way optimization design significantly enlarges the achievable downlink-uplink rate region compared with the two heuristic designs. It is also shown that phase-averaging is superior to time-sharing when the number of RIS elements is large.